1. Field of the Invention
The present invention relates to a signal receiver, a method of controlling the signal receiver, and a GPS device utilizing the signal receiver and method.
2. Description of the Related Art
Global Navigation Satellite Systems (GNSSs) for receiving signals from satellites orbiting the Earth and determining the current positions of the respective objects have been widely used. In particular, among the GNSS, Global Positioning Systems (GPSs) have been commonly known.
In practice, the GPS receiver is designed to obtain positional information by analyzing satellite waves received from a plurality of GPS satellites (for example, four or more satellites) via GPS antennas, calculate the current position of the GPS navigation system based on the positional information, and output the positional data denoting the current position to the navigation device. In other words, the GPS receiver receives signals from the respective satellites and demodulates the received signals to obtain the satellite orbit information. Then, the GPS receiver can derive the three-dimensional position of the GPS receiver using simultaneous equations from the GPS satellite's orbit and time information and the received signal's delay time. Any influence of a difference between the internal clock of the GPS receiver and the clock of the satellite can be removed by receiving signals from four or more GPS satellites.
In order to inform the user of the current position of the GPS receiver at every predetermined time interval (e.g., every one second), it is convenient to continuously track a signal from the GPS satellite. Generally, a signal to be used by the GPS receiver for positioning calculation is referred to as an L1 band C/A (Clear and Acquisition or Coarse and Acquisition) code. In other words, the signal is a GPS signal obtained by modulating a carrier of 1575.42 MHz with a binary phase shift keying (BPSK) scheme using a signal obtained by directly spreading data of 50 bps (bit per second) with a pseudo random noise (PRN) code having a code length of 1023 and a chip rate of 1.023 MHz. Therefore, synchronization of PRN codes, carriers, and data is desired to allow the GPS receiver to receive signals from the GPS satellites. A typical GPS receiver, which has been used in the art, uses a phase-locked loop (PLL) for GPS signal carrier synchronization and the number of PLLs should correspond to the number of signals to be received.
As mentioned above, to allow a typical GPS receiver to perform positioning calculation using signals from GPS satellites, it is desirable to receive the signals from four or more GPS satellites. Therefore, at least four independent PLLs are desirable. In the following description, GPS satellites having signals thereof that can be received at a particular area on the earth are referred to as visible satellites and the number of such satellites is referred to as the “number of visible satellites”. The number of visible satellites is 10 or more in an open sky area but based on season, time, latitude, longitude, and terrain (see, for example, B. Hofmann-Wellenhof, H. Lichtenegger, and J. Collins “Global Positioning System: Theory and Practic”, Springer-Verlag, Tokyo. 2005). If visible satellites can be simultaneously tracked as much as possible, the frequency that the number of the satellites from which signals is not more than four can be decreased even when a part of the receiving signals being tracked is blocked by buildings and geographic features. This results in a decrease in frequency of interrupting the positioning calculation due to a decrease in the number of receivable satellites. A component that performs synchronization-holding channel processing to receive one signal from one GPS satellite will be referred to as a “synchronization-holding channel” in the following description. Typically, in many cases, the GPS receiver may have eight or more synchronization-holding channels.